docs: switch changelog skill to markdown sections

[jcllobet]

Summary

• update .opencode/skills/changelog-release-tracker/SKILL.md so it generates per-release markdown sections instead of a markdown table
• define the exact section shape for each release entry, including booleans, counts, user-facing bullets, and blank publication fields
• require the tracker file to stay sorted oldest to newest so new releases append naturally

Validation

• git diff --check
• python3 -c \"from pathlib import Path; text=Path('.opencode/skills/changelog-release-tracker/SKILL.md').read_text(); assert 'markdown sections, not a table' in text; assert '## vX.Y.Z' in text; assert 'oldest to newest' in text; print('validated Factory changelog skill format')\"

Notes

• Factory-only skill update
• the OpenWork tracker content is updated in the separate OpenWork PR